A memory cell of a three-dimensional memory is commonly of a metal/oxide/nitride/oxide/silicon (MONOS) type. A MONOS type memory cell includes a channel semiconductor layer that is a silicon layer, a tunnel insulating film that is an oxide film, a charge trapping film that is a nitride film, a block insulating film that is an oxide film, and a control gate that is a metal layer.
A MONOS type memory cell includes, as a charge storage layer, a charge trapping film instead of a floating gate. A floating gate generally includes a semiconductor layer such as a polysilicon layer and a conductive layer, such as a metal layer, while a charge trapping film generally includes an insulating film, such as a silicon nitride film. A MONOS type three-dimensional memory cell is advantageous in that the thickness of the charge trapping film can be smaller than the thickness of the floating gate, but suffers from a low erase speed and an increase in interference between memory cells when the memory cell is miniaturized.
Therefore, suggested is forming a floating gate and an inter-gate insulating film between the tunnel insulating film and the charge trapping film in the MONOS type three-dimensional memory cell. In this case, storing charges in the charging trapping film and not in the floating gate allows the floating gate to have a small thickness. Furthermore, using a high-k film as the charge trapping film allows a large amount of charge to be stored in the charge trapping film. However, a problem arises in that charges in the charge trapping film are likely to leak to the floating gate. That is, a three-dimensional memory cell, including a floating gate and a charge trapping film, suffers from a low charge retaining ability, through having a high charge storing ability.